The viewing screen of a color image display, such as a CRT of the aperture-mask type, may be constituted of spaced elemental image areas of luminescent material that are selectively excited to luminescence to produce an image. One expedient used to improve the contrast of the luminescent image that is produced on the screen is a light-absorbing matrix adjacent the elemental areas of the screen. Such a matrix has the effect of substantially reducing the intensity of the ambient light that is reflected from the spaces between the elemental image areas of the screen.
Image displays including a light-absorbing matrix, methods for preparing the matrix, and materials constituting the matrix are disclosed, for example, in issued U.S. Pat. Nos. 3,558,310 to E. E. Mayaud, 4,049,452 to E. M. Nekut and 4,556,820 to R. P. Thompson. A preferred method for preparing a matrix, referred to as reverse printing, includes photographically producing a stencil of organic polymeric material on a support, coating the support with a slurry of particulate light-absorbing material, drying the coating and then removing the stencil with the overlying coating while leaving the coating in the open areas of the stencil in place. Luminescent materials are then deposited in the open areas of the matrix where the stencil was removed, after which the structure is subjected to at least a first baking in air at temperatures above 400.degree. C. and a second baking in air at temperatures above 300.degree. C.
Of all the matrix materials suggested previously, matrices of colloidal graphite and of carbon black have been the most used and the most successful. Both materials leave much to be desired. Colloidal graphite, although low in cost and relatively resistant to oxidation upon subsequent baking in air, is gray and less light-absorbing than carbon black, and produces matrices with less-than-desirable resolution due to relatively large average particle size in the range of 0.1 to 5.0 microns. Carbon black, although it is more light-absorbing and has a much smaller average particle size (in the range of 0.009 to 0.070 micron) than colloidal graphite, is much less resistant to oxidation upon subsequent baking in air than is colloidal graphite. Excessive oxidation of a carbon-black matrix results in poor light absorption, or requires the deposition of excessive amounts of carbon black to compensate for the material lost by oxidation.
It is desirable to provide a display comprising a matrix of light-absorbing particles that has lower cost, smaller particle-size-range and greater absorbing power of ambient light than previously-used colloidal graphite, and also greater resistance to oxidation upon subsequent baking in air than previously-used carbon black. The light-absorbing particles should produce aqueous slurries with good storage properties by ordinary factory processes, and should be compatible with prior matrix-making processes. The novel method satisfies these desirable characteristics.